The process of friction stir welding is well known and is especially useful for welding a butt joint formed between adjacent metal pieces, for example, aluminum alloys, copper alloys, etc. The process uses a nonconsumable rotating tool comprised of a pin, often threaded, extending from a shoulder. To effect a weld, the rotating pin is forced into the joint and the adjacent metal pieces until a surface of the shoulder contacts the upper surfaces of the workpieces. The friction of the rotating shoulder on the metal pieces plasticizes an annular region of the metal around the pin. The rotating tool is then moved along the joint; and as the pin is moved along the joint, the pressure provided by the leading face of the pin forces hot, plasticized metal to the back of the pin where it fills the void left by the moving pin. After cooling and hardening, the weld left is a fine grained, hot worked joint that is very strong and resistant to breaking.
The friction stir welding process presents several challenges to a machine structure. For example, in moving the rotating tool toward and away from the weld joint and along the weld joint, known drive mechanisms, for example, a screw drive, may be utilized. However, the process of sinking the nonconsumable rotating pin into the solid metal of the weld joint requires a very large force that must be maintained while the pin is traversed along the joint between the metal pieces. A screw drive as well as most other types of drives often found on a machine are normally used to control position and velocity of the member being moved. Using such drives to control force is substantially more complicated. For example, a strain gage or other force measuring device must be implemented to provide a force feedback signal; and a force control loop is then used to control the operation of the screw drive such that the desired force is achieved and maintained. Providing a force control with such known drives is further complicated by force induced deflections that occur in the structure of the machine. Thus, obtaining the desired control over force with known drives is complex and expensive. Therefore, there is a need to provide a machine design by which not only is the position of the rotating tool controlled but the force applied to the tool is also controlled using simple, reliable and inexpensive components.
Another issue in the design of a stir welding machine is how to securely clamp the metal pieces during the welding process. Of particular concern is how to secure the edges of the metal pieces forming the joint to be welded. As will be appreciated, the plunging of the rotating tool into the metal pieces and the high friction forces created by the rotating shoulder on the surfaces of the metal pieces create forces tending to separate the pieces. Further, the heat generated in the process often results in the edges of the metal pieces bending or warping. Further, the thicknesses of the two metal pieces are often not absolutely identical resulting in the weld process tending to work the thicker metal piece more. As a result, an elaborate system of clamps is often used in which a series of clamps is located on both sides of the joint over its entire length. In other applications, a pair of rollers is rigidly connected to, and rotate with, the rotating tool, thereby continuously circling the tool as it is moved along the joint. As will be appreciated, if one metal piece is thicker than the other piece, the circling rollers are constantly hitting the raised edge of the thicker metal piece which may result in undesirable vibrations and wear. Further, with rigid roller axles, as one roller moves up and over the thicker metal piece, the other roller is lifted slightly from the thinner metal piece; and the roller contacting the thicker metal piece applies a greater force than the roller contacting the thinner metal piece. Thus, there is a need for a simple, reliable and inexpensive joint clamping mechanism that applies equal forces to both of the metal pieces even if one piece is thicker than the other piece.
The friction stir welding process produces substantial heat in the metal pieces as well as in the rotating tool and adjacent machine components. The heat may be significant enough to adversely affect the performance or life of components adjacent the rotating tool, for example, the spindle bearings. Therefore, there is a need to provide a friction stir welding machine that limits the transfer of heat from the rotating tool and the welding area to other components of the welding machine.
It is common to clamp two metal pieces, weld a joint, clamp another piece and weld another joint. However, in many applications, it is more efficient to clamp more than two pieces on the welding machine at one time and then, successively weld each of the joints without having to handle individual pieces between the welds. In such an application, it is important that none of the metal pieces move during the welding process, so that all of the joints remain in their desired position ready to be welded. Thus, there is also a need for a relatively simple but effective system for clamping metal pieces to be welded in their desired positions. The work holding system should not only properly clamp the joint being welded, but an effective work holding system should permit a number of metal pieces to be mounted and clamped on a worktable so that a number of joints can be successively welded with minimum handling of the pieces.